The present invention relates generally to electrical safety devices and more particularly to electrical safety devices for a power cable.
Conventional electrical appliances typically receive alternating current (AC) power from a power source, such as an electrical outlet, through a power cable. The power cable enables the electrical appliance, or load, to receive from the power source the current necessary to operate.
A power cable typically comprises at least two conducting lines through which current travels from the power source to the load. Specifically, a power cable typically comprises a power line and a neutral line. A metal sheath can be used to surround the power line and the neutral line in order to provide the power cable with arc sensing capabilities.
The connection of an electrical appliance to a power supply through a pair of conducting lines can create a number of potentially dangerous conditions. In particular, there exists the risk of ground fault and grounded neutral conditions in the conducting lines. A ground fault condition occurs when there is an imbalance between the currents flowing in the power and neutral lines. A grounded neutral condition occurs when the neutral line is grounded at the load.
Ground fault circuit interrupters are well known in the art and are commonly used to protect against ground fault and grounded neutral conditions. A ground fault circuit interrupter (GFCI) typically comprises a differential transformer with opposed primary windings, one primary winding being associated with the power line and the other primary winding being associated with the neutral line. If a ground fault condition should occur on the load side of the GFCI, the two primary windings will no longer cancel, thereby producing a flux flow in the core of the differential transformer. This resultant flux flow is detected by a secondary winding wrapped around the differential transformer core. In response thereto, the secondary winding produces a trip signal which, in turn, serves to open at least one of the conducting lines between the power supply and the load, thereby eliminating the dangerous condition.
As an example, in U.S. Pat. No. 5,757,598, to V. V. Aromin, there is disclosed a ground fault circuit interrupter (GFCI) which interrupts the flow of current through a pair of lines extending between a source of power and a load. The GFCI includes a circuit breaker having a switch located in one of the pair of the lines. The switch has a first position in which the source of power in its associated line is not connected to the load and a second position in which the source of power in its associated line is connected to the load. A relay circuit is coupled to the switch for selectively positioning the switch in either the first or second position. The relay circuit includes a solenoid which operates in either an energized or a de-energized state. When energized, the solenoid positions the switch in its second position and when de-energized, the solenoid positions the switch in its first position. The GFCI also includes a booster circuit for selectively supplying a first voltage through the switch and to the solenoid which is sufficient to cause the solenoid to switch from its de-energized state to its energized state. A power supply circuit supplies a second voltage to the solenoid which is less than the first voltage. The second voltage is sufficient to maintain the solenoid in its energized state after being initially energized by the first voltage but is insufficient to switch the solenoid from its de-energized state to its energized state.
A latch circuit operable in first and second bi-stable states allows the solenoid to switch from its de-energized state to its energized state and remain in its energized state when in its first bi-stable state and allowing solenoid to switch from its energized state to its de-energized state and remain in its de-energized state when in its second bi-stable state. A fault detection circuit detects the presence of a fault condition in at least one of the lines extending between the power and the load and causes the latch circuit to latch in its second bi-stable state upon detection of the fault condition.
While GFCI circuits of the type described above are well known and widely used in commerce to protect against ground fault and grounded neutral conditions, it should be noted that a power cable is susceptible to other types of hazardous conditions which are not protected against by a conventional GFCI circuit.
As an example, it has been found that one type of arcing condition can occur between one of the conducting lines and the metal sheath which surrounds the conducting lines. It should be noted that the presence of this type of arcing condition between either the power line and the metal sheath or the neutral line and the metal sheath can result in a fire or other dangerous condition, which is highly undesirable.
Accordingly, in U.S. Pat. No. 4,931,894 to R. Legatti, there is disclosed a ground fault current interrupter circuit (GFCI) which is provided with the additional capacity of detecting and protecting against arcing between a power line and the metal sheath or cover of a power cable. An arc protection winding is located on the core of the GFCI differential transformer and is connected in series with a resistance between the metal sheath and a neutral or return line. By adjusting the number of turns of the arc protection winding and the size of the series resistance, the sensitivity of the arc protection arrangement to arcing current may be set at a desired level.
Although well known in commerce, the GFCI disclosed in Legatti suffers from a notable drawback. Specifically, the GFCI disclosed in Legatti requires a differential transformer in order to detect arcing conditions between the power line and the metal sheath or the neutral line and the metal sheath. As can be appreciated, the implementation of a differential transformer significantly increases the overall size and cost of the product, which is highly undesirable.
It is an object of the present invention to provide a new and improved safety circuit for a power cable which includes two or more conducting lines and a metal sheath surrounding the conducting lines.
It is another object of the present invention to provide a safety circuit as described above which senses the presence of an arcing condition between one of the conducting lines and the metal sheath, and in response thereto, opens at least one of the conducting lines between the power supply and the load.
It is yet another object of the present invention to provide a safety circuit as described above which may be mass produced, has a minimal number of parts, and can be easily assembled.
Accordingly, in one embodiment of the present invention, there is provided a fireguard circuit for use with a power cable, said power cable connecting a power source with a load, said power cable comprising a power line, a neutral line and a metal sheath which surrounds the power line and the neutral line, said fireguard circuit comprising a circuit breaker comprising a first switch located in one of said lines between the power source and the load, said switch having a first position in which the power source in its associated line is connected to the load and a second position in which the power source in its associated line is not connected to the load, a circuit opening device for setting said switch in either its first position or its second position, said circuit opening device being operable in either a first state or a second state, said circuit opening device setting said switch in its first position when in its first state and said circuit opening device setting said switch in its second position when in its second state, a first silicon controlled rectifier (SCR) for detecting the presence of an arcing condition between the power line and the metal sheath, said first SCR setting said circuit opening device at its second state upon detecting the presence of an arcing condition between the power line and the metal sheath, and a second silicon controlled rectifier (SCR) for detecting the presence of an arcing condition between the neutral line and the metal sheath, said second SCR setting said circuit opening device at its second state upon detecting the presence of an arcing condition between the neutral line and the metal sheath.
Additional objects, as well as features and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.